Various types of furnaces have been used in the past to recover aluminum from dross and skimmings. In the conventional aluminum recovery procedure, salt is initially introduced into a rotating furnace or kiln and heated to the molten state. Aluminum dross and skimmings are then added to the furnace and melted. The molten aluminum stratifies as a layer beneath the molten salt, and the salt layer protects the molten aluminum from oxidation. After the materials have been melted, rotation of the furnace is stopped and the molten aluminum is withdrawn from a tap hole in an end of the furnace. After withdrawal of the molten aluminum, the salt is similarly withdrawn from the end of the furnace by gravity.
The conventional aluminum recovery furnace is cylindrical in shape and in order to drain or withdraw the molten aluminum and salt from the furnace, a hydraulically actuated tilt mechanism is employed to tilt the furnace. As the furnace has substantial weight, up to 60,000 lbs. the tilt mechanism is complex and adds considerable cost to the unit.
Other commonly used furnaces have an outer cylindrical drum and a tapered refractory lining. By use of the tapered refractory lining, the molten materials can be drained from the furnace without the necessity of a tilt mechanism.
To rotate the conventional furnace, a pair of guide rings and a gear ring are welded in spaced relation to the outer cylindrical surface of the furnace. As the furnace has a substantial diameter, generally in the range of 8 to 10 ft., it is very difficult to precisely position the guide rings and drive ring on the outer surface of the furnace with the result that a certain amount of eccentricity inherently results. The eccentricity produces a substantial load on the bearings and also increases the energy requirement for rotating the furnace.
As a further disadvantage, the guide rings and gear ring, which are welded to the outer metal shell of the furnace, tend to restrict the thermal expansion of the shell during the heating process. As expansion of certain areas of the metal shell is restricted, the refractory lining in these areas tends to crack and deteriorate, thereby resulting in serious maintenance problems.